The pre-processing directives provide the ability to conditionally skip sections of source files, to report error and warning conditions, and to delineate distinct regions of source code. The term "pre-processing directives" is used only for consistency with the C and C++ programming languages. In C#, there is no separate pre-processing step; pre-processing directives are processed as part of the lexical analysis phase.
A preprocessor directive must be the only instruction on a line. Preprocessing directives are lines in your program that start with `#'. Whitespace is allowed before and after the `#'. The `#' is followed by an identifier that is the directive name. For example, `#define' is the directive
Types are:
#if, #else, #elif, #endif, #define, #undef, #warning, #error, #line, #region, #endregion

They are used for:
Conditional compilation
Line control
Error and Warning reporting

For example u can refere the MS site ...
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/csspec/html/vclrfcsharpspec_2_5_4.asp

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